1. Field of the Invention
The invention relates to a turbocharger installed in an engine, such as an automobile engine, and a method of manufacturing the turbocharger.
2. Description of the Related Art
A conventional engine such as an automobile engine, for example, may be installed with a turbocharger to achieve improvements in engine output and so on. The turbocharger turns exhaust gas from the engine into a power supply using a turbine wheel, and supercharges the engine.
There is a great need to reduce a heat capacity of the turbocharger in order to improve the cleanliness of the exhaust gas. The reason for this is as follows. A catalyst device that purifies the exhaust gas using a catalyst such as a nitrogen oxide (NOx) reduction catalyst may be provided in the engine on a downstream side of the turbocharger with respect to the exhaust gas flow. When the temperature of the catalyst in the catalyst device is within a predetermined range, the catalyst is activated such that the exhaust gas is purified efficiently.
Hence, when the heat capacity of the turbocharger is large, it is difficult to increase the temperature of the catalyst that purifies the exhaust gas from a state in which the temperature of the catalyst must be raised to an activation temperature (a temperature at which the catalyst is activated), for example during engine startup or the like. In other words, the temperature of the catalyst in the catalyst device is raised by heat from the exhaust gas passing through the turbocharger, and therefore, when the heat capacity of the turbocharger is large, a large amount of heat is lost from the exhaust gas, thereby making it difficult to raise the temperature of the catalyst.
As regards the heat capacity of the turbocharger, the heat capacity of a housing (turbine housing) that forms an exhaust gas passage for leading the exhaust gas from the engine into the turbine wheel has a particularly large effect on increases in the temperature of the exhaust gas. Therefore, as the heat capacity of the turbine housing, which serves as an exhaust system component of the engine, decreases, the amount of heat lost from the exhaust gas decreases, making it possible to raise the temperature of the catalyst to the activation temperature quickly such that the cleanliness of the exhaust gas is improved.
Various proposals have been made in the related art for reducing the heat capacity of a turbocharger. One of these proposals is a technique for reducing the thickness of the turbine housing or forming the turbine housing from sheet metal (to be referred to simply as “reducing the thickness of the turbine housing” hereafter). The thickness of the turbine housing can be reduced by forming the turbine housing from a thin plate material obtained by press-molding a heat-resistant material such as stainless steel (SUS) sheet metal, for example. However, reducing the thickness of the turbine housing may lead to a reduction in the rigidity of the turbine housing. A reduction in the rigidity of the turbine housing leads to a reduction in the performance (turbo efficiency) and reliability of the turbocharger and a reduction in the durability of the turbine housing.
More specifically, a turbine housing having reduced rigidity caused by a reduction in thickness is more likely to undergo thermal deformation due to thermal expansion and thermal contraction occurring during a thermal cycle that accompanies an operation of the engine, thermal deformation in constitutional components of the engine or peripheral components (a support stay and so on) of the turbocharger, vibration input during an operation of the engine, and so on. When thermal deformation occurs in the turbine housing, a gas leak may occur due to deformation of a gas seal portion of the turbine housing, an increase in clearance between the turbine housing and another member, and so on. When this type of gas leak occurs in the turbocharger, the performance and reliability of the turbocharger deteriorate. Furthermore, when thermal deformation occurs during an operation of the engine or the like in a turbine housing having reduced rigidity caused by a reduction in thickness, or the turbine housing deforms due to thermal deformation of a peripheral component, it becomes difficult to secure sufficient durability in the turbine housing.
To solve these problems accompanying a reduction in the thickness of the turbine housing, a turbocharger may be provided with a member (to be referred to hereafter as a “reinforcement member”) that reinforces the turbine housing (see Japanese Patent Application Publication No. 2008-106667 (JP-A-2008-106667) and Japanese Patent Application Publication No. 2008-121470 (JP-A-2008-121470), for example). In other words, the rigidity of a housing main body, i.e. the part of the turbine housing formed from a thin plate material, is secured by the reinforcement member. The reinforcement member takes an overall substantially annular shape, and is provided about a rotary axis center of the turbocharger so as to surround a housing main body forming an exhaust gas passage.
More specifically, the reinforcement member includes a pair of ring-shaped annular portions provided about the rotary axis center of the turbocharger at an interval in a rotary axis direction, and a columnar connecting portion that connects the annular portions. The turbine housing is formed by fixing the annular portions on either side to the housing main body by welding or the like such that the reinforcement member is fixed to the housing main body.
In a conventional reinforcement member provided on a housing main body constituted by a thin plate material, constitutional components of the pair of annular portions and the connecting portion that connects the pair of annular portions are formed integrally by welding or casting. More specifically, in a turbocharger disclosed in JP-A-2008-106667, a pair of flanges constituting the pair of annular portions of the reinforcement member are integrated by being welded to a connecting ring constituting the connecting portion of the reinforcement member. JP-A-2008-106667 also states that the pair of flanges constituting the annular portions may be manufactured as an integral cast component including the connecting portion. In a turbocharger disclosed in JP-A-2008-121470, a pair of base portions constituting the pair of annular portions and a connecting portion that connects the pair of base portions are likewise formed integrally by casting.
However, when the reinforcement member of the turbine housing is a cast component or an integral welded structure, the following problems arise. When the reinforcement member is constituted by a cast component, the surface roughness of a surface thereof is likely to increase, leading to an increase in loss of a fluid (exhaust gas) flowing over a wall surface of the reinforcement member. Furthermore, a cast component must be machined to compensate for a lack of precision in the formed material, leading to an increase in cost. Meanwhile, when the reinforcement member is constituted by an integral welded structure, welding processes must be performed, thereby increasing the complexity of the processing. Moreover, in the case of a welded structure, welding distortion leads to a reduction in precision, causing an increase in cost.
Further, it has been learned through experiments and the like that an external force acting on the reinforcement member due to thermal deformation of the housing main body and peripheral components during an engine operation and so on is mainly constituted by a load that acts in a direction for compressing the reinforcement member in the rotary axis direction of the turbocharger or a load (a stretching force) that acts in a direction for pulling the reinforcement member in the rotary axis direction. This external force input into the reinforcement member increases the amount by which the reinforcement member deforms, leading to a reduction in the fatigue life of the reinforcement member.